Don't warn about copilot-max-char during normal Emacs functions

[Aaron Madlon-Kay]

I get warnings like the below when I update packages via e.g. package-update-all.

⛔ Warning (copilot):  *temp*-529187 size exceeds 'copilot-max-char' (100000), copilot completions may not work

I am activating copilot-mode in prog-mode-hook, so I assume that it's catching large Elisp files that are opened at package install time. I do want copilot-mode in regular Elisp files, so I don't think it's feasible to be more selective about what hook I put it on.

Would it make sense to have copilot ignore buffers with the *temp*- prefix?

Originally posted by @amake in #311 (comment)